1. Field of the Invention
The present invention is in the field of elements for use in construction, and, more particularly, is in the field of construction elements for reducing damage caused to structures during seismic events, severe wind events and other forces applied to structures.
2. Description of the Related Art
During a seismic event, such as an earthquake, or during a severe wind event, a structure may be subjected to large forces which can result in severe damage or total destruction of the structure. Conventional walls of a residential structure comprise a lower mudsill that rests on a concrete footing or other suitable foundation. An upper double top plate is spaced apart from the mudsill with a plurality of vertical studs which are generally evenly spaced (e.g., sixteen inches or twenty-four inches on center (“o/c”)). The outer portion of a conventional wall is sheathed with plaster, siding or other suitable material, and the inner portion is covered with gypsum board, paneling or the like. Such a conventional wall cannot withstand the forces of seismic event or severe wind event because the shape of the wall distorts when the upper portion of the conventional wall moves laterally with respect to the mudsill. Even if the structure withstands the seismic event, the lateral movement of the wall causes cracks, broken windows and the like. In many cases, the wall does not return to its original shape after the seismic event or wind event is over.
In order to reduce the likelihood of structural damage during a seismic event or wind event, many residential structures are now constructed with shear walls. In particular, at least a portion of each of the inner and outer walls comprises a shear wall. A shear wall may comprise a specially constructed section of any wall which is constructed at a building site. Alternatively a shear wall may comprise a panel constructed separately and inserted into any wall at the building site. Both types of shear walls are included within the scope of the following description.
Unlike a conventional wall, a shear wall includes a solid structural sheet positioned over the outer surface or the inner surface. The solid structural panel of the shear wall may advantageously comprise one or more plywood sheets of suitable thickness. Alternatively, the shear wall may comprise a laminated panel of steel or another metallic material. See, for example, U.S. Pat. No. 5,768,841 to Swartz et al. for Wallboard Structure. Each end of the shear wall comprises a larger vertical member (e.g., an end post) to which the solid structural sheet is also attached. For example, the end posts may advantageously comprise a conventional 4×4 or larger post. During seismic events or severe wind events, the forces applied to the shear wall are coupled to the foundation via the end posts. Furthermore, the end posts are secured to the foundation via hold down devices, such as, for example, the hold down connector shown in U.S. Pat. No. 5,249,404 to Leek et al. for Holdown Structure.
The solid structural sheet of the shear wall inhibits the movement of the upper double top plate with respect to the mudsill when force is applied. Thus, the shear wall does not distort. By tying the remaining portion of any wall to the shear wall, movement of the entire wall is inhibited, and damage caused by the force is substantially reduced.
Although shear walls reduce the damage during seismic events, studies have shown that during very large seismic events or severe wind events, the forces applied to the shear wall and coupled to the end posts are sufficiently large to cause the lower ends of the end posts to compress the mudsill. The wood fibers in the compressed mudsill are crushed to reduce the thickness of the mudsill. The reduced thickness of the mudsill allows more movement of the shear wall, and thus may result in severe damage or destruction of the structure.
Because of the compression of the mudsill, building codes have been revised recently to require the mudsill of the shear wall to be constructed from larger material. For example, instead of allowing a contractor to use a conventional 2×4 or 2×6 material having a nominal thickness of 1.5 inches, the contractor is required to use a 3×4 or 3×6 mudsill having a nominal thickness of 2.5 inches to substantially reduce compression of the mudsill of the shear wall.
The additional thickness of the mudsill would appear to be a relatively straightforward way of reducing damage caused by seismic events and severe wind events; however, the thicker mudsill causes additional construction expenses for a contractor. For example, three-inch thick lumber is non-conventional. Thus, a contractor has to special order 3×4 or 3×6 lumber to construct the mudsill or create the mudsill at the construction site from larger material. In addition, the conventional studs between the mudsill and the double top plate have to be cut to be one inch shorter than conventional studs. Although this might appear to be a minor inconvenience, it should be understood that hundreds or thousands of studs are used at a large number of construction sites (e.g., at a new housing development or a new apartment complex). The additional time required to cut each stud rather than using the studs as delivered from the lumber supplier adds substantial cost and waste to a large construction project. Furthermore, since the required thickness of the 3×4 or 3×6 mudsill is 2.5 inches thick, the carpenters building walls with such mudsills must use larger nails to connect the mudsill to the bottoms of the studs. The larger nails are more expensive. In addition, the larger nails do not work with conventional nail guns. Since the economies of modern construction depend on the use of nail guns as well as other power tools to reduce construction time, the loss of the use of the nail gun for such repetitive work has a significant economic impact on the profit of the contractor or the cost to the owner of the finished structure. Thus, an alternative to the thicker mudsill is needed.